1. Field of the Invention:
This invention relates to an exposure control device for an optical apparatus of the kind arranged to drive an exposure amount adjusting member by means of a stepping motor.
2. Description of the Related Art:
The arrangement and operation of the automatic exposure control device of the conventional video camera are described below with reference to FIG. 5:
The incident light of an image taken from an object through a lens optical system 1 comes to iris blades 2. The quantity of the incident light is adjusted by the iris blades 2 and, after that, the incident light is imaged on the image pickup plane of an image sensor 3. The object image thus formed on the image sensor 3 is photo-electrically converted into a picked-up image signal. The picked-up image signal is supplied to a camera signal processing circuit 4. The camera signal processing circuit 4 then performs various processes such as gamma conversion, etc., to obtain a chrominance signal C and a luminance signal Y.sub..gamma. as video signals. These signals are supplied to a camera encoder 5 to be converted into an NTSC signal. The NTSC signal thus obtained is outputted to the outside from a camera part in the form of a composite video signal or the like.
Meanwhile, a luminance signal Y outputted from the camera signal processing circuit 4 is supplied to an integrating detector circuit 6 to be used for generating a control signal for controlling the iris blades 2 in such a way as to give an exposure apposite to the luminance condition of the image plane. The output of the integrating detector circuit 6 is taken in by a camera microcomputer 7. At the camera microcomputer 7, a value obtained by the integrating detection process of the integrating detector circuit 6 is compared with a reference value (an apposite exposure value). A signal indicating a difference between the two values is outputted to a driver 9. The driver 9 then converts the signal from the camera microcomputer 7 into an apposite voltage. The voltage is applied to an actuator 10. The actuator 10 drives the iris blades 2 accordingly.
Further, an encoder 11 is arranged to detect the position of the iris blades 2. Information on the position of the iris blades 2 thus detected is supplied to the camera microcomputer 7 and to an AF (automatic focusing) microcomputer.
In the automatic exposure control device of the video camera described, the actuator 10 which is arranged to drive the iris blades 2 is an electromagnetic motor of an analog type. The driver 9 which is arranged to drive the electromagnetic motor is also composed of an analog circuit. A control system for controlling the motor is formed as a closed-loop control system. In such a closed-loop control system that is arranged to operate by using analog signals, inadequate matching between control elements tends to cause an instability phenomenon such as oscillations, hunting or the like of the iris blades which is being controlled. To avoid such instability phenomena as oscillations, hunting and the like, the dynamic characteristic and frequency response of an iris mechanism of the camera must be constant. However, the dynamic characteristic and frequency response of the iris mechanism of one camera somewhat differ from those of another camera. The conventional automatic exposure control device, therefore, tends to cause such instability phenomena as oscillations, hunting and the like to take place in the iris blades. The oscillations or hunting tends to take place particularly when a difference between the luminance signal and the reference luminance value is small, i.e., accordingly as the position of the iris blades comes near to a desired value.